Contrasting Summer Monsoon Cold Pools South of Indian Peninsula Presented at

Contrasting Summer Monsoon Cold Pools South of Indian Peninsula

Presented at

ROMS/TOMS Asia-Pacific Workshop-2009, Sydney Institute of Marine Sciences, Sydney, Australia

on 31st March 2009

By

Francis P. A. and M. Ravichandran

MOG, INCOIS, Hyderabad

ROMS/TOMS Asia-Pacific Workshop-2009, SIMS, Sydney, Australia. 31st March 2009

What is Summer Monsoon Cold Pool?

Average TMI SST for July-August 2002

Summer Monsoon Cold Pool

The mechanism proposed for the formation of the SMC is the coastal upwelling due to strong monsoon wind.


TMI SST Patterns for July/August 2002 and 2003

Intensity of SMC varies both in interannual and intraseasonal time scales


TMI SST Difference Patterns (2002-2003)

JULY AUGUST

The variation in the intensity of SMC is attributed to the variation in the strength of upwelling due to the variation of forcing


Difference in the wind filed (July 2002-July2003)

This difference is too small to produce such a difference in the cooling


OLR Anomaly patterns for July 2002 and July 2003

July 2002 July 2003

The convection pattern in July 2002 and July 2003 are opposite over the equatorial Indian Ocean

Such a scenario was present in may occasions, and it is known as Equatorial Indian Ocean Oscillation (EQUINOO) [S. Gadgil, P. N. Vinayachandran, P. A. Francis and S. Gadgil, GRL, 2004]


Hypothesis

The signature of such an oscillation is seen in many atmospheric and oceanic parameters

When it is coupled with the ocean, then it is referred as Indian Ocean Dipole mode [Saji et al 1999, Webster et al 1999]

The differential heating of the atmospheric column due to anomalous convection associated with the EQUINOO leads to anomalous easterlies/westerlies over the equator

Difference in the wind filed over the tropical Indian Ocean

(July 2003-July2002)


Hypothesis

•The zonal wind along the equatorial Indian Ocean was dramatically different in the boreal summer months of 2002 and 2003

Zonal wind averaged over 70-90E, 2.5oS-2.5oN


Strong easterlies along the equatorial Indian Ocean trigger upwelling Kelvin waves, which propagate to the east and downwelling Rossby waves which propagate to the west

The off-equatorial maxima of downwelling (upwelling) can deepen (shoalen) the isothermal layers and decrease (increase) the efficiency of surface cooling

EQ upwelling

Downwelling

Downwelling

upwelling

Downwelling

upwelling

EQ

easterlies

westerlies


We show, with the help of ROMS simulations, that it is this downwelling that prevented the summer monsoon cooling (south of peninsnular India) in July 2003.

Further, by August 2003, the zonal wind along the equator became westerly.

This lead to the triggering of downwelling Kelvin waves and upwelling Rossby waves in the equatorial Indian Ocean which in turn lead to intensification of SMC

Hence, in addition to the local forcing, the interannual and intraseasonal variation in the intensity of summer monsoon cold pool is related to the processes over the equatorial Indian Ocean.


A supporting evidence of this hypothesis has come from the difference in SSH patterns between July 2002 and July 2003

Difference in SSH anomaly (TOPEX) patterns (July2002- July2003)


This hypothesis needs to be tested with a “GOOD” Ocean General Circulation Model, which can simulate not only the coastal features, but also the basin scale features

It is essential to make sure that the equatorial wind forcing influence the subsurface thermal structure in the SMC region

We use ROMS for confirming our hypothesis


Model set up

Major CPP options chosen:

1. Bulk parameterization2. EMINUSP computed3. Prescribed net long wave radiation4. Eastern and southern boundaries are relaxed to climatology5. Mellor-Yamada mixing scheme6. Salinity correction with a time scale of 15 days

Domain: 30-160E, 30S-30N

Horizontal resolution:0.25 x 0.25 degree

Vertical levels :40Theta_s:10Theta_b :0Hc :100m

Spin-up : 10yrs

Interannual : from 2000 onwards


Forced with

Wind fields : QSCAT

Air temperature : NCEP-Reanalysis

Relative humidity : NCEP-Reanalysis

Precipitation : CMAP

Net longwave Radiation : NCEP-Reanalysis

Net shortwave Radiation : NCEP-Reanalysis

Sea level pressure : NCEP-Reanalysis

All forcing fields except precipitation are daily mean

precipitation field is monthly mean.


Validation

Temperature profiles at 90oE, 1.5oS compared with PMEL-RAMA buoy profiles


Timeseries of Temperature and current at 1.5m depth at 90oE, 1.5oS

Validation

Temperature

Zonal Current


Simulated summer monsoon cold pools in July/August 2002/2003


Difference Patterns of surface temperature (2002-2003) for July/August


Difference in the dynamic height (zeta) anomaly between Jul2002 and Jul2003


Difference in vertical velocity (2002-2003) at 65m in July and August


Evolution of the difference in the vertical structure of temperaturein the cold pool region (4-8oN, 75-83oE)

July August

8

20

30

43

60

90

150


Conclusions:

The observed intraseasonal/interannual variation in the intensity of the summer monsoon cold pool is closely linked to the positive/negative phase of the equatorial Indian Ocean oscillation (EQUINOO)

The easterlies along the central equatorial Indian Ocean (CEIO) during a positive EQUINOO trigger upwelling Kelvin waves to the east and downwelling Rossby waves to the west. The off-equatorial maxima of downwelling prohibit effective surface cooling

It is essential to consider the basin wide processes in order understand the coastal processes in the Indian Ocean domain. Hence any GCM for the coastal forecasting system in the Indian Ocean should have a domain, at least covering the entire basin.


Important References:

1. Rao et al 2006, 2007(GRL)

2. Shankar et al 2008, (J. Earth Sys. Sci.)

3. Gadgil et al 2003, 2007 (Cur. Sci.)

4. Gadgil et al 2004 (GRL)

5. Saji et al 1999, (Nature)

6. Webster et al 1999, (Nature)

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Contrasting Summer Monsoon Cold Pools South of Indian Peninsula Presented at